1. Field of the Invention
The present invention relates to an active matrix substrate composed of a substrate having transistors formed thereon, and more particularly to an active matrix substrate to be suitably used for a display device.
2. Description of the Related Art
Currently, display devices such as liquid crystal display devices and organic EL display devices incorporating active matrix substrates are in wide use.
As active matrix substrates, those employing thin film transistors (TFTs) as active elements are mainly used (hereinafter referred to as “TFT substrates”). With the improvement in display quality in recent years, further optimization of TFT structures is being studied.
For example, Japanese Laid-Open Patent Publication No. 2002-190605 discloses a structure for suppressing fluctuations in the parasitic capacitance of a TFT. In FIG. 3 of Japanese Laid-Open Patent Publication No. 2002-190605, for example, there is described a TFT including a semiconductor layer which partially spreads outside a gate electrode. This TFT is structured so that, within the region of the semiconductor layer spreading outside, a portion which is located over a source electrode and a portion which is located over a drain electrode are shielded by a gate electrode.
However, the inventors have conducted a study to find that the aforementioned conventional structure has a problem in that the display quality is likely to be affected by the precision of the patterning which the semiconductor layer is subjected to. The reason behind this is that, as will be specifically described later with reference to comparative examples, within the region of the semiconductor layer spreading outside the gate electrode, a portion which is located over a source electrode and a portion which is located over a drain electrode are connected via a semiconductor layer whose potential is not controlled by the gate electrode, thus allowing a leak current to occur between the source electrode and the drain electrode.
Moreover, in a structure where a semiconductor layer is formed so as to overlay a gate insulating layer at an intersection between a source bus line and a gate bus line, this being in order to prevent short-circuiting between the source bus line and the gate bus line, an unwanted residual semiconductor film may be left, possibly causing a leak current associated with the residual semiconductor film. Such a leak current also degrades display quality.